Synthesis of block polymers obtained by controlled free radical polymerization

ABSTRACT

The invention concerns a method for preparing a first generation polymer comprising a step which consists in free radical polymerization of a composition comprising: at least an ethylenically unsaturated monomer, a source of free radicals, and at least a cyclic organic compound including at least a tetrathiophosphate group.

The present invention relates to a novel process of free radicalpolymerization which provides block polymers, and to the block polymersthus obtained.

Block polymers are commonly prepared by ionic polymerization. This typeof polymerization has the drawback of allowing the polymerization onlyof certain types of apolar monomers, especially styrene and butadiene,and of requiring a particularly pure reaction medium and temperaturesoften lower than ambient, in order to minimize side reactions, therebyresulting in severe operational constraints.

Free radical polymerization has the advantage of being easy to implementwithout observing excessive purity conditions, and at temperatures ofambient or above. Until recently, however, there was no free radicalpolymerization process allowing block polymers to be obtained.

In conventional free radical polymerization, the reactivity of thegrowing macroradicals is nonselective: the chains undergo irreversibletermination by coupling or dismutation. As a consequence, it is verydifficult to control the structure of the chains. The possibilities forobtaining functional polymers, telechelic polymers or block copolymersare extremely limited. Recently, a new process of free radicalpolymerization has been developed: this is “controlled” or “living” freeradical polymerization. A number of techniques have been developed inwhich the polymeric chain ends can be reactivated by virtue of areversible termination or transfer reaction (dormant species/activespecies equilibrium).

Controlled free radical polymerization presents the followingdistinctive aspects:

1. the number of chains is fixed throughout the reaction;

2. the chains all grow at the same rate, which results in:

-   -   a linear increase in molecular masses with conversion,    -   a narrowed mass distribution;

3. the average molecular mass is controlled by the molar ratio ofmonomer to chain precursor;

4. the possibility of preparing block copolymers.

The controlled character is all the more marked given that the rate ofreactivation of the chains to radical is very great, ahead of the rateof chain growth (propagation). Cases exist where this is not always true(i.e., the rate of reactivation of the chains to radical is lower thanthe propagation rate) and conditions 1 and 2 are not observed;nevertheless, it is still possible to prepare block copolymers.

Several approaches have been described for controlling free radicalpolymerization by reversible termination reaction. The most commonlycited consists in introducing into the medium counterradicals whichreversibly combine with the growing macroradicals, such as for examplethe nitroxyl radicals (Georges et al., Macromolecules, 26, 2987,(1993)). This technique is characterized by high temperatures for makingthe C—O bond labile. Another method called Atom Transfer RadicalPolymerization involves salts of transition metals combined with organicligands and an initiator which is generally an organic halide; thecontrol of polymerization is made possible by the reversible cleavage ofthe C-Halogen bond (Matyjaszewski K., patent application WO 96/30421). Adrawback of this polymerization is that a large quantity of metalremains in the reaction medium at the end of polymerization. Otsu (Otsuet al., Makromol. Chem. Rapid Comm., 3, 127-132, (1982), Otsu et al.ibid, 3, 123-140, (1982), Otsu et al., Polymer Bull., 7, 45, (1984),ibid, 11, 135, (1984), Otsu et al., J. Macromol. Sci. Chem., A21, 961,(1984), Otsu et al., Macromolecules, 19, 2087, (1989)), have shown thatsome organic sulfides, particularly dithiocarbamates, made it possibleunder UV irradiation to cause chains to grow in a controlled manner. Theprinciple is based on the photolysis of the C—S bond which regeneratesthe carbon macroradical, on the one hand, and the dithiocarbamylradical, on the other hand. The controlled character of the reaction isdue to the reversibility of the C—S bond under UV irradiation. It isthus possible to obtain block copolymers. By contrast, the equilibriumconstant for activation/deactivation is not very favorable in the lightof the speed of propagation, which has the consequence of generatingrelatively wide molecular mass distributions. Thus, the polydispersityindex (I_(p)=M_(w)/M_(n)) is between 2 and 5 (Otsu et al., 25, 7/8,643-650, (1989)).

As regards the methods based on a reversible transfer, there may bementioned iodine transfer polyermization (ITP) developed by Tatemoto(Tatemoto et al., EP 272698, Daikin Industries Ltd.), more recentlyrediscovered by Matyjaszewski and renamed “Degenerative Transfer”(Matyjaszewski et al., Macromolecules, 28, 2093 (1995)). PCT patentapplications WO 98/01478 in the name of DuPont de Nemours and WO99/35178 in the name of Rhodia Chimie describe the use of transferagents which are reversible by addition-fragmentation, of the RSC=SR′dithioester type, for the synthesis of copolymers possessing controlledarchitecture. Another family of reversible transfer agents, thexanthates RSC=SOR′, were described in patent application WO 98/58974 inthe name of Rhodia Chimie as precursors of block copolymers. The controlof free radical polymerization by dithiocarbamates RS(C=S)NR₁R₂ has alsorecently been described in the patent applications WO 99/35177 in thename of Rhodia and WO 99/31144 in the name of DuPont de Nemours.

Controlled free radical polymerization possesses an advantage overconventional free radical polymerization when the aim is to preparechains which are of low molecular weight and are functionalized(reactive telomers). Such polymers are required for specificapplications such as, for example, coatings and adhesives.

Thus, when the aim is to synthesize chains grafted with on average 2functional comonomers, the fraction of chains having one functional siteat most becomes great when the average degree of polymerization is lessthan a threshold value (e.g. 20 or 30). Controlled free radicalpolymerization makes it possible to reduce or even inhibit the formationof these oligomers to zero or one functional site, which are detrimentalto application performance.

In the remainder of the description, the term “polymer”, is used todescribe homopolymers or copolymers, unless indicated otherwise.

Moreover, a block polymer is understood to be a copolymer comprising atleast two successive enchainments of blocks of monomer units ofdifferent chemical constitution. The blocks may consist of a homopolymeror of a polymer obtained from a mixture of ethylenically unsaturatedmonomers. In this case the block may be a random copolymer. The blockcopolymer may comprise two blocks each composed of random copolymers. Inthis case, the ethylenically unsaturated monomers are such that theblocks obtained are different in nature. By differences in nature aremeant blocks composed of monomers of different types, but also blockscomposed of monomers of the same type but in different amounts.

One aim of the present invention is to provide a novel process of freeradical polymerization using a novel control agent.

A second aim of the invention is to provide a process of polymerizationduring which the number average molar masses M_(n) of the polymersobtained are effectively controlled, these number average molar massesM_(n) varying upwards with the conversion of the monomer.

Another aim is to provide a controlled free radical polymerizationprocess for synthesizing chain end functionalized polymers.

These aims and others which will appear in the remainder of thedescription are achieved by the present invention, which provides aprocess for preparing a first generation functionalized polymer, whichcomprises a step of free radical polymerization of a compositioncomprising:

-   -   at least one ethylenically unsaturated monomer,    -   a source of free radicals, and    -   at least one cyclic organic compound comprising at least one        tetrathiophosphate group.

By tetrathiophosphate group is meant a group of formula:

More particularly, the cyclic organic compound comprising at least onetetrathiophosphate group is chosen from a compound of formula (IA)(tetraphosphorus decasulfide or P₄S₁₀) and a compound of general formula(IB):

-   -   where A represents an optionally substituted, linear or branched        alkyl radical, or a group chosen from the groups of the        following formulae (IIA) and (IIB):    -   with R₁, which are identical or different, represents an        optionally substituted alkyl, aryl, aralkyl, alkene or alkyne        group,    -   and where R, which is different or identical, represents:        -   an optionally substituted alkyl, aryl, aralkyl, alkene or            alkyne group,        -   an optionally substituted, aromatic, saturated or            unsaturated heterocycle or carbon ring,        -   a polymeric chain,        -   a radical of formula (IIIA):        -   a radical of formula (IIIB):    -   with m, which is identical or different, an integer greater than        or equal to 2.

Advantageously, m is less than or equal to 5.

According to the present invention, the optionally substituted alkyl,aryl, aralkyl, alkene or alkyne groups generally have 1 to 20 carbonatoms, preferably 1 to 12, and more preferably 1 to 9 carbon atoms. Theymay be linear or branched. They may also be substituted especially byoxygen atoms, in the form especially of esters, sulfur or nitrogenatoms.

Among the alkyl radicals, mention may be made especially of the methyl,ethyl, propyl, butyl, pentyl, isopropyl, tert-butyl, pentyl, hexyl,octyl, decyl or dodecyl radical.

The alkene radicals are radicals generally of 2 to 10 carbon atoms; theyhave at least one ethylenic unsaturation, such as the vinyl or allylradical.

The alkyne groups are radicals generally of 2 to 10 carbon atoms; theyhave at least one acetylenic unsaturation, such as the acetylenylradical.

Among the aryl radicals, mention may be made especially of the phenylradical, optionally substituted especially by a nitro or hydroxylfunctional group.

Among the aralkyl radicals mention may be made especially of the benzylor phenethyl radical, optionally substituted especially by a nitro orhydroxyl functional group.

When R is a heterocycle, the piperidine, morpholine, pyrrolidone orpiperazine radical is preferred.

When R is a polymeric chain, this polymeric chain may result from anionic or free radical polymerization or from a polycondensation.

The group R, when it is substituted, may be substituted by substitutedphenyl groups, substituted aromatic groups, saturated or unsaturatedcarbon rings, saturated or unsaturated heterocycles, or by the followinggroups: alkoxycarbonyl or aryloxycarbonyl (—COOR′), carboxyl (—COOH),acyloxy (—O₂CR′), carbamoyl (—CONR′₂), cyano (—CN), alkylcarbonyl,alkylaryl-carbonyl, arylcarbonyl, arylalkylcarbonyl, phthalimido,maleimido, succinimido, amidino, guanidino, hydroxyl (—OH), amino(—NR′₂), halogen, perfluoroalkyl C_(n)F_(2n+1), allyl, epoxy, alkoxy(—OR′), S-alkyl, S-aryl, groups exhibiting an ionic or hydrophiliccharacter, such as alkali metal salts of carboxylic acids, alkali metalsalts of sulfonic acid, polyalkylene oxide chains (PEO, POP), cationicsubstituents (quaternary ammonium salts), R′ representing an alkyl oraryl group or a polymeric chain.

In one particular embodiment, R is a substituted or unsubstituted,preferably substituted, alkyl group.

When A represents an optionally substituted alkyl radical, the preferredalkyl radical is the radical of formula

in which m is an integer greater than or equal to 2.

Advantageously, the maximum value of m is 5.

Preferably, the groups R₁ in a compound of formula (IB) are identical.

Preferably, the groups R and R₁ in a compound of formula (IB) areidentical.

The compound (IB) useful in the present invention is, for example, acompound of formula (IB) in which R is chosen from:

-   -   C₆H₅    -   CH₂C₆H₅    -   CH(CH₃)(C₆H₅)    -   C(CH₃)₂(C₆H₅)    -   C(CH₃)₂CN.

Among the compounds of formulae (IB), the compounds of the followingformulae (IB1), (IB2), (IB3), (IB4) and (IB5) may be especially chosen:

The compounds of formulae (IA) and (IB) have the advantage of beingreadily available. The compound of formula (IA) and certain reagents offormula (IB), such as the compound2,4-bis(methylthio)-2,4-dithioxo-1,3,2,4-dithiadiphosphetane, arecommercial products. Certain reagents of formula (IB) may especially beobtained by reaction between the compound of formula (IA) with at leastone alcohol (ROH) or at least one thiol (RSH) (H. Davy, J. Chem. Soc.,Chem. Commun., 457 (1982); G. Ohms et al. J. Chem. Soc. Dalton Trans.1297 (1995); M. Démarcq J. Chem. Soc. Dalton Trans. 2221 (1988)).

In all cases the process of the invention is implemented in the presenceof a source of free radicals; however, for certain monomers, such asstyrene, the free radicals which allow the polymerization to beinitiated may be generated by the ethylenically unsaturated monomeritself at sufficiently high temperatures, generally greater than 100° C.In this case it is not necessary to add a source of additional freeradicals.

The source of free radicals useful in the process of the presentinvention is generally a free radical polymerization initiator. The freeradical polymerization initiator may be selected from the initiatorsconventionally used in free radical polymerization. This may be, forexample, one of the following initiators:

-   -   hydrogen peroxides such as: tertiary butyl hydro-peroxide,        cumene hydroperoxide, t-butyl peroxyacetate, t-butyl        peroxybenzoate, t-butyl peroxyoctoate, t-butyl        peroxyneodecanoate, t-butyl peroxyisobutarate, lauroyl peroxide,        t-amyl peroxypivalate, t-butyl peroxy-pivalate, dicumyl        peroxide, benzoyl peroxide, potassium persulfate, ammonium        persulfate;    -   azo compounds such as: 2-2′-azobis(isobutyro-nitrile),        2,2′-azobis(2-butanenitrile), 4,4′-azobis(4-pentanoic acid),        1,1′-azobis(cyclohexanecarbonitrile),        2-(t-butylazo)-2-cyanopropane,        2,2′-azobis[2-methyl-N-(1,1)-bis(hydroxymethyl)-2-hydroxyethyl]propionamide,        2,2′-azobis(2-methyl-N-hydroxyethyl]propionamide,        2,21-azobis(N,N′-dimethyleneisobutyramidine) dichloride,        2,2′-azobis(2-amidinopropane) dichloride,        2,2′-azobis-(N,N′-dimethyleneisobutyramide),        2,2′-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide),        2,2′-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide),        2,2′-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide],        2,2′-azobis(isobutyramide) dihydrate,    -   redox systems comprising combinations such as:        -   mixtures of hydrogen peroxide, alkyl peroxide, peresters,            percarbonates and the like and of any one of the iron salts,            titanous salts, zinc formaldehyde-sulfoxylate or sodium            formaldehyde-sulfoxylate, and reducing sugars;        -   alkali metal or ammonium persulfates, perborate or            perchlorate in combination with an alkali metal disulfite,            such as sodium metabisulfite, and reducing sugars;        -   alkali metal persulfate in combination with an            arylphosphinic acid, such as benzenephosphonic acid and            other similar acids, and reducing sugars.

In accordance with one embodiment, the amount of initiator to be used isdetermined such that the amount of radicals generated is not more than50 mol %, preferably not more than 20 mol %, relative to the quantity ofcyclic organic compound comprising at least one tetrathiophosphategroup.

The ethylenically unsaturated monomers useful in the process of thepresent invention are all monomers which polymerize in the presence ofthe cyclic organic compound comprising at least one tetrathiophosphategroup to give active polymeric chains.

These ethylenically unsaturated monomers are for example

-   -   styrene and styrene derivatives such as alpha-methylstyrene or        vinyltoluene,    -   vinyl esters of carboxylic acid, such as vinyl acetate, vinyl        Versatate®, vinyl propionate,    -   vinyl and vinylidene halides,    -   ethylenically unsaturated monocarboxylic and dicarboxylic acids,        such as acrylic acid, methacrylic acid, itaconic acid, maleic        acid, fumaric acid, and the monoalkyl esters of the dicarboxylic        acids of the type mentioned with alkanols having preferably from        1 to 4 carbon atoms, and their N-substituted derivatives,    -   amides of unsaturated carboxylic acids, such as acrylamide,        methacrylamide, N-methylolacrylamide or -methacrylamide,        N-alkylacrylamides,    -   ethylenic monomers containing a sulfonic acid group and its        alkali metal or ammonium salts, for example vinylsulfonic acid,        vinylbenzenesulfonic acid, alpha-acrylamidomethylpropanesulfonic        acid, 2-sulfoethylene methacrylate,    -   amides of vinylamine, particularly vinylformamide or        vinylacetamide,    -   unsaturated ethylenic monomers containing a secondary, tertiary        or quaternary amino group or a heterocyclic group containing        nitrogen, such as, for example, vinylpyridines, vinylimidazole,        aminoalkyl (meth)acrylates and aminoalkyl-(meth)acrylamides,        such as dimethylaminoethyl acrylate or methacrylate,        ditert-butylaminoethyl acrylate or methacrylate,        dimethylaminomethyl-acrylamide or -methacrylamide, or        zwitterionic monomers such as, for example,        sulfopropyl-(dimethyl)aminopropyl acrylate,    -   dienes, for example butadiene, chloroprene,    -   (meth)acrylic esters,    -   vinyl nitriles,    -   vinylphosphonic acid and its derivatives.

The use of the following monomers is preferred:

-   -   styrene and styrene derivatives such as alpha-methylstyrene or        vinyltoluene,    -   vinyl and vinylidene halides,    -   vinyl nitriles,    -   dienes, for example butadiene or chloroprene,    -   unsaturated ethylenic monomers containing a secondary, tertiary        or quaternary amino group or a heterocyclic group containing        nitrogen, such as, for example, vinylpyridines and        vinylimidazole.

By (meth)acrylic esters are meant the esters of acrylic acid and ofmethacrylic acid with hydrogenated or fluorinated C₁-C₁₂, preferablyC₁-C₈, alcohols. Among the compounds of this type mention may be madeof: methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate,isobutyl acrylate, 2-exhylhexyl acrylate, t-butyl acrylate, methylmethacrylate, ethyl meth-acrylate, n-butyl methacrylate, isobutylmethacrylate.

The vinyl nitriles include more particularly those having from 3 to 12carbon atoms, such as acrylonitrile and methacrylonitrile in particular.

For the preparation of polyvinylamine blocks it is preferred to use asethylenically unsaturated monomers the amides of vinylamine, forexample, vinylformamide or vinylacetamide. The polymer obtained is thenhydrolyzed at acidic or basic pH.

For the preparation of polyvinyl alcohol blocks it is preferred to useas ethylenically unsaturated monomers the vinyl esters of carboxylicacid, such as vinyl acetate, for example. The polymer obtained is thenhydrolyzed at acidic or basic pH.

The types and amounts of polymerizable monomers employed in accordancewith the present invention vary depending on the particular end use forwhich the polymer is intended. These variations are well known and canbe readily determined by the skilled worker.

These ethylenically unsaturated monomers may be used alone or inmixtures.

In accordance with one specific embodiment, in the process for preparinga first generation polymer, the ethylenically unsaturated monomercorresponds to the formula (VIA): CXX′ (=CV−CV′)_(b)=CH₂, the firstgeneration polymer obtained comprises n times the unit of formula (IV):

with

-   -   n is greater than or equal to 1, preferably greater than 6,    -   V, V′, which are identical or different, represent: H, an alkyl        group or a halogen,    -   X and X′, which are identical or different, represent H, a        halogen or a group R₂, OR₂, O₂COR₂, NHCOH, OH, NH₂, NHR₂,        N(R₂)₂, (R₂)₂N⁺O⁻, NHCOR₂, CO₂H, CO₂R₂, CN, CONH₂, CONHR₂ or        CON(R₂)₂, in which R₂ is selected from alkyl, aryl, aralkyl,        alkylaryl, alkene or organosilyl groups which are optionally        perfluorinated and optionally substituted by one or more        carboxy, epoxy, hydroxyl, alkoxy, amino, halogen or sulfonic        groups, and    -   b is 0 or 1.

The polymerization may be conducted in bulk, in solution; in emulsion,in dispersion or in suspension. It is preferably implemented in solutionor in emulsion.

The process is preferably implemented semicontinuously.

The temperature may vary between the ambient temperature and 150° C. inaccordance with the nature of the monomers used.

In general, during the polymerization, the instantaneous polymer contentwith respect to the instantaneous amount of monomer and polymer, isbetween 50 and 99% by weight, preferably between 75 and 99%, morepreferably still between 90 and 99%. This content is maintained, in aknown way, by controlling the temperature, the rate of addition of thereactants and, where appropriate, the polymerization initiator.

The process is generally implemented in the absence of a UV source, bythermal initiation.

In accordance with one variant of the invention, at least one acidicorganic compound may be added to the composition of the processaccording to the invention.

This variant makes it possible especially to obtain polymers having alow polydispersity index (M_(w)/M_(n)), in general not more than 2,preferably not more than 1.5, Mw being the molecular mass by weight.

More particularly, the acid used is a compound of formula R₃COOH, inwhich formula R₃ represents an optionally substituted alkyl, aryl,aralkyl, alkene or alkyne group, an optionally substituted aromatic,saturated or unsaturated heterocycle or carbon ring.

The process of the invention can be implemented starting from a mixtureof ethylenically unsaturated monomers. In that case a random firstgeneration polymer is obtained. By selecting monomers of particulartypes, for example, hydrophilic monomers and hydrophobic monomers, andthe amount of each of these monomers in the block, a block is obtainedwhich has particular properties. This procedure is particularlyadvantageous when the first generation polymer thus obtained is anintermediate in the preparation of a block copolymer.

The present invention likewise provides a process for preparing an Nthgeneration block copolymer by free radical polymerization, N beinggreater than or equal to 2, which comprises:

-   -   a first step of free radical polymerization as described above,        in order to form the first generation polymer, followed by    -   N-1 steps of free radical polymerization, each of these steps        being implemented starting from a composition as described above        comprising:        -   at least one ethylenically unsaturated monomer,        -   a source of free radicals, and        -   the block polymer obtained in the preceding step of free            radical polymerization, the ethylenically unsaturated            monomer or monomers being such that the block formed in this            step is different in nature to the block formed in the            preceding step.

For example, a second generation block copolymer can be obtained by aprocess which comprises the free radical polymerization of a compositioncomprising:

-   -   at least one ethylenically unsaturated monomer,    -   a source of free radicals, and    -   the first generation polymer obtained by free radical        polymerization of the composition comprising the cyclic organic        compound comprising at least one tetrathiophosphate group and        ethylenically unsaturated monomers, the block thus obtained        being different in nature to the first generation polymer.

In accordance with one embodiment of the invention, (1) a firstgeneration polymer is synthesized starting from a composition comprisingone or more ethylenically unsaturated monomers, a source of freeradicals and a cyclic organic compound comprising at least onetetrathiophosphate group, and then (2) the first generation polymerobtained in step (1) is used to prepare a diblock (second generation)copolymer by contacting this first generation polymer with one or moreethylenically unsaturated monomers and a source of free radicals, theblock obtained in step (2) being different in nature to the firstgeneration polymer of step (1).

This step (2) may be repeated with further monomers and the diblockcopolymer obtained in order to synthesize a new block and to obtain atriblock copolymer.

Thus it is possible to repeat as many times as necessary the step ofpolymerization starting from a block copolymer to give a copolymerhaving an additional block.

The process of the invention therefore makes it possible to obtain adiblock copolymer comprising two blocks of formula (V):

starting from a composition comprising:

-   -   an ethylenically unsaturated monomer of formula        (VIB):CYY′(CW=CW′)_(a)=CH₂,    -   a first generation polymer as described above,    -   n and n′, which are identical or different, are greater than or        equal to 1,    -   V, V′, W and W′, which are identical or different, represent: H,        an alkyl group or a halogen,    -   X, X′, Y and Y′, which are identical or different, represent H,        a halogen or a group R₂, OR₂, O₂COR₂, NHCOH, OH, NH₂, NHR₂,        N(R₂)₂, (R₂)₂N⁺O⁻, NHCOR₂, CO₂H, CO₂R₂, CN, CONH₂, CONHR₂ or        CON(R₂)₂, in which R₂ is selected from alkyl, aryl, aralkyl,        alkaryl, alkene or organosilyl groups which are optionally        perfluorinated and optionally substituted by one or more        carboxy, epoxy, hydroxyl, alkoxy, amino, halogen or sulfonic        groups, and    -   a and b, which are identical or different, are 0 or 1.

The ethylenically unsaturated monomers which are useful are thosedescribed above.

In accordance with this process for preparing block polymers, when it isdesired to obtain polymers having blocks which are homogeneous and donot have a composition gradient, and if all the successivepolymerizations are conducted in the same reactor, it is essential thatall of the monomers used in one step have been consumed before thepolymerization of the following step begins, i.e. before the nextmonomers are introduced.

When the desire is to obtain a random block, the polymerization step isimplemented with a composition containing a mixture of ethylenicallyunsaturated monomers.

The present invention further provides first generation polymers andblock polymers obtainable by any one of the processes of the invention.These polymers have a controlled molecular mass.

In accordance with one specific embodiment, the block polymers compriseat least two polymeric blocks selected from the following combinations:

-   -   polystyrene/poly-p-methylstyrene,    -   polystyrene/polymethyl acrylate,    -   polystyrene/polyethyl acrylate,    -   polystyrene/poly-tert-butyl acrylate,    -   polystyrene/polyvinylpyridine,    -   polystyrene/polybutadiene

One of the blocks may also be composed of a random copolymer obtainedstarting from a mixture of ethylenically unsaturated monomers.

The following examples illustrate the invention without, however,limiting its scope.

EXAMPLES Example 1 Synthesis of Compounds of Formula (IB) Example 1.1Synthesis of2,4-bis(phenylthio)-2,4-dithioxo-1,3,2,4-dithiadiphosphetane (IB2)

The method is derived from the reference H. Davy, Sulfur Letters, vol.3(2) 39, (1985). P₄S₁₀ (44.4 g, 0.1 mol) is mixed with 150 ml ofdichlorobenzene until this solvent refluxes (160° C.). Phenylthiol (47ml) is slowly added to the solution over 90 minutes, while increasingthe temperature up to 190° C. The solution becomes clear; the reactiontemperature is reduced to 100° C. The stirring is stopped in order toallow the residual P₄S₁₀ to be separated by settling. The solution isfiltered in the hot state. The product crystallizes after filtration.The yield is 60%.

¹H NMR (CS₂): 7.3-7.7 ppm: phenyl group. ³¹p NMR (CS₂): 22.9 ppm.

Example 1.2 Synthesis of2,4-bis(benzylthio)-2,4-dithioxo-1,3,2,4-dithiadiphosphetane (IB1)

The method is derived from the reference H. Davy, Sulfur Letters, vol.3(2) 39, (1985). P₄S₁₀ (8.88 g, 0.02 mol) is mixed with 30 ml ofdichlorobenzene until this solvent refluxes (160° C.). Benzylthiol (16.5ml; 0.14 mol) is slowly added to the solution over 40 minutes, whileincreasing the temperature up to 190° C. The solution becomes clear; thereaction temperature is then reduced to 80° C. The stirring is stoppedin order to allow the residual P₄S₁₀ to be separated by settling. Thesolution is filtered in the hot state. The product crystallizes slowlyafter filtration. The yield is 50%.

Example 2 Synthesis of the Polymers

In the examples given below, the polymerization reactions are carriedout in Schlenk-type apparatus. In each case, the mixture present in thereactor is connected to a vacuum ramp, immersed in liquid nitrogen,followed by three cycles of freezing-vacuum-return to ambienttemperature in order to degas the mixture. The reactor is subsequentlyleft under nitrogen at the appropriate temperature. Kinetic monitoringis carried out by taking samples of the reaction medium over time, undera stream of nitrogen. The monomer conversion is determined by gravimetryfollowing evaporation of the residual monomer under vacuum.

The (co)polymers are analyzed by steric exclusion chromatography (SEC)using THF as elution solvent; the molar masses are expressed inpolystyrene equivalents (g.mol⁻¹).

These examples demonstrate that the free radical polymerization of theethylenically unsaturated monomers is controlled, employing thetetrathiophosphate-bearing cyclic precursors. Polymerization control isdemonstrated in particular through the increase in the number averagemolar masses (Mn) with the monomer conversion.

Example 2.1 Polystyrene Prepared from P₄S₁₀

0.015 g (3.33×10⁻⁵ mol) of P₄S₁₀, 5.5 mg (3.33×10⁻⁵ mol) of AIBN and4.54 g (4.37×10⁻² mol) of styrene are mixed. After degassing, thereaction medium is heated to 60° C. The change in number average molarmass with the conversion of the styrene is indicated in table 1 below.

TABLE 1 Mass polymerization of styrene initiated by AIBN in the presenceof P₄S₁₀: [S]/[P₄S₁₀] = 1313, [AIBN]/[P₄S₁₀] = 1, T = 60° C. Sample Time(h) Conversion (%) M_(n) × 10⁻³ (g/mol) 1 1 1 8 2 3 3 14 3 7 6 22 4 2115 43 5 29 20 44 6 48 28 50

Example 2.2 Polystyrene Prepared from2,4-dithioxo-2,4-bis(methylthio)-1,3,2,4-dithiadiphosphetane (IB3)

0.125 g (4.39×10⁻⁴ mol) of precursor2,4-dithioxo-2,4-bis(methylthio)-1,3,2,4-dithiadiphosphetane, 7.5 mg(4.57×10⁻⁵ mol) of AIBN and 4.54 g (4.37×10⁻² mol) of styrene are mixed.After degassing, the solution is heated to 60° C. The change in numberaverage molar mass with the conversion of the styrene is indicated intable 2 below.

TABLE 2 Mass polymerization of the styrene initiated by AIBN in thepresence of 2,4-dithioxo-2,4-bis-(methylthio)-1,3,2,4-dithiadiphosphetane: [S]/[IB3] = 100, [AIBN]/[IB3]= 0.1, T = 60° C. Sample Time Conversion (%) M_(n) × 10⁻³ (g/mol) 1  2 h15 min 10 3.9 2  4 h 40 min 17 9 3 22 h 50 min 40 25 4 44 h 60 70

Example 2.3 Polystyrene Prepared in the Presence of2,4-dithioxo-2,4-bis(benzylthio)-1,3,2,4-dithiadiphosphetane (IB1) andBenzoic Acid

0.114 g (2.62×10⁻⁴ mol) of precursor2,4-dithioxo-2,4-bis(benzylthio)-1,3,2,4-dithiadiphosphetane (IB1), 67mg (5.5×10⁻⁴ mol) of benzoic acid are mixed in 3 ml of toluene at 110°C. for one hour.

After returning to ambient temperature, 17.2 mg (1.04×10⁻⁴ mol) of AIBNand 6 ml (5.24×10⁻² mol) of styrene are added. After degassing, thesolution is heated to 60° C. The change in number average molar masswith the conversion of the styrene is indicated in table 3 below:

TABLE 3 Mass polymerization of the styrene initiated by AIBN in thepresence of 2,4-dithioxo-2,4-bis(benzylthio)-1,3,2,4-dithiadiphosphetane and benzoic acid: [S]/2[IB1]= 100; [C₆H₅COOH]/[IB1] = 2.1; [AIBN]/ 2[IB1] = 0.2; T = 60° C. SampleTime Conversion (%) M_(n) × 10⁻³ (g/mol) 1 1 h 15 min 5 0.4 2  4 h 150.5 3 7 h 30 min 22 1.2 4 23 h 45 4

Example 2.3 Homopolystyrene Prepared in the Presence of2,4-dithioxo-2,4-bis(phenylthio)-1,3,2,4-dithiadiphosphetane (IB2) andBenzoic Acid

37 mg (9.21×10⁻⁵ mol) of precursor2,4-dithioxo-2,4-bis(phenylthio)-1,3,2,4-dithiadiphosphetane (IB2), 20mg (5.5×10⁻⁴ mol) of benzoic acid and 3 ml (2.62×10⁻² mol) of styreneare mixed. After degassing, the solution is heated to 110° C. The changein number average molar mass with the conversion of the styrene isindicated in table 4 below:

TABLE 4 Mass polymerization of the styrene initiated by AIBN in thepresence of 2,4-dithioxo-2,4-bis(phenylthio)-1,3,2,4-dithiadiphosphetane and benzoic acid:[S]/2[IB2] = 142; [C₆H₅COOH]/[IB2] = 1.85; T = 110° C. Sample Time (h)Conversion (%) M_(n) × 10⁻³ (g/mol) 1 15 15 1.2 2 44 24 5.6 3 111 70 11

1. A process for preparing a first generation polymer, which comprises astep of free radical polymerization of a composition comprising: atleast one ethylenically unsaturated monomer, a source of free radicals,and at least one cyclic organic compound comprising at least onetetrathiophosphate.
 2. The process of claim 1, wherein the cyclicorganic compound comprising at least one tetrathiophosphate group isselected from the group consisting of a compound of formula (IA)(tetraphosphorus decasulfide or P₄S₁₀) and a compound of general formula(IB):

where A represents a substituted or non-substituted, linear or branchedalkyl radical, or a group selected from the groups consisting of thefollowing formulae (IIA) and (IIB):

with R₁, which are identical or different, represents a substituted ornon-substituted alkyl, aryl, aralkyl, alkene or alkyne group, and whereR, which is different or identical, represents: a substituted ornon-substituted alkyl, aryl, aralkyl, alkene or alkyne group, asubstituted or non-substituted, aromatic, saturated or unsaturatedheterocycle or carbon ring, a polymeric chain, a radical of formula(IIIA):

a radical of formula (IIIB):

with m an integer greater than or equal to
 2. 3. The process of claim 2,wherein the compound of formula (IB) is selected from the groupconsisting of compounds of formula (IB) with A representing a group offormula (IIA) or (IIB), in which the groups R and R₁ are identical. 4.The method as claimed in claim 2, wherein the cyclic organic compoundcomprising at least one tetrathiophosphate group is a compound offormula (IB) in which A represents a radical of formula:

in which m is an integer greater than or equal to
 2. 5. The process ofclaim 2, wherein the compounds of formula (IB) are selected from thegroup consisting of compounds of the following formulae (IB1) to (IB5):

in which m is an integer greater than or equal to
 2. 6. The process asclaimed in claim 1, wherein the composition further comprises at leastone acidic organic compound.
 7. The process as claimed in claim 1, inwhich the ethylenically unsaturated monomer corresponds to the followingformula (VIA):CXX′(=CV−CV′)_(b)=CH₂, and the first generation polymer obtainedcomprises n times the unit of formula (IV):

with n is greater than or equal to 1, V, V′, which are identical ordifferent, represent a hydrogen atom, an alkyl group or a halogen, X andX′, which are identical or different, represent H, a halogen or a groupR₂, OR₂, O₂COR₂, NHCOH;⁻OH, NH₂, NHR₂, N(R₂)₂, (R₂)₂N⁺O⁻, NHCOR₂, CO₂H,CO₂R₂, CN, CONH₂, CONHR₂ or CON(R₂)₂, in which R₂ is selected fromalkyl, aryl, aralkyl, alkaryl, alkene or organosilyl groups which areoptionally perfluorinated and optionally substituted by one or morecarboxy, epoxy, hydroxyl, alkoxy, amino, halogen or sulfonic groups, andb is 0 or
 1. 8. A process for preparing an Nth generation blockcopolymer by free radical polymerization, N being greater than or equalto 2, which comprises: a first step of free radical polymerization inorder to form a first generation polymer from a composition comprising:at least one ethylenically unsaturated monomer, a source of freeradicals, and at least one cyclic organic compound comprising at leastone tetrathiophosphate group, a number N-1 steps of free radicalpolymerization, each of these steps being implemented starting from acomposition comprising: at least one ethylenically unsaturated monomer,a source of free radicals, and the block polymer obtained in thepreceding step of polymerization, the ethylenically unsaturated monomeror monomers being such that the block formed in this step is differentin nature to the block formed in the preceding step.
 9. The process asclaimed in claim 8, for preparing a second generation block copolymerwhich comprises the free radical polymerization of a compositioncomprising: at least one ethylenically unsaturated monomer, a source offree radicals, and the first generation polymer.
 10. The process asclaimed in claim 8, for preparing a block copolymer comprising twoblocks of formula (V):

starting from a composition comprising: an ethylenically unsaturatedmonomer of formula (VIB): CYY′ (CW=CW′)_(a)=CH₂, said first generationpolymer, n and n′, which are identical or different, are greater than orequal to 1, V, V′, W and W′, which are identical or different,represent: H, an alkyl group or a halogen, X, X′, Y and Y′, which areidentical or different, represent H, a halogen or a group R₂, OR₂,O₂COR₂, NHCOH, OH, NH₂, NHR₂, N(R₂)₂, (R₂)₂N⁺O⁻, NHCOR₂, CO₂H, CO₂R₂,CN, CONH₂, CONHR₂or CON(R₂)₂, in which R₂ is selected from alkyl, aryl,aralkyl, alkaryl, alkene or organosilyl groups which are optionallyperfluorinated and optionally substituted by one or more carboxy, epoxy,hydroxyl, alkoxy, amino, halogen or sulfonic groups, and a and b, whichare identical or different, are 0 or
 1. 11. The process as claimed inclaim 1, in which the at least one ethylenically unsaturated monomer isselected from the group consisting of styrene, alpha-methylstyrene,vinyltoluene, vinyl halides, vinyl nitriles, dienes, unsaturatedethylenic monomers containing a secondary, tertiary or quaternary aminogroup and a heterocyclic group containing nitrogen.
 12. A polymerobtainable by the process as defined in claim
 1. 13. The polymer asclaimed in claim 12, which has a polydispersity index of not more than2.
 14. The polymer obtainable by the process of claim 10 which has atleast two polymeric blocks.
 15. The polymer as claimed claim 14, inwhich the at least two polymeric blocks are selected from the followingcombinations: polystyrene/poly-p-methylstyrene, polystyrene/polymethylacrylate, polystyrene/polyethyl acrylate, polystyrene/poly-tert-butylacrylate, polystyrene/polyvinylpyridine, or polystyrene/polybutadiene.16. The polymer as claimed in claim 14, in which at least one of theblocks consists of a random polymer obtained from a mixture ofethylenically unsaturated monomers.
 17. The process as claimed in claim7, where n is greater than
 6. 18. The process as claimed in claim 3,wherein the cyclic organic compound comprising at least onetetrathiophosphate group is a compound of formula (1B) in which Arepresents a radical of the formula:

in which m is an integer greater than or equal to
 2. 19. The process asclaimed in claim 2, wherein the composition further comprises at leastone acidic organic compound.
 20. The process as claimed in claim 3,wherein the composition further comprises at least one acidic organiccompound.
 21. The process as claimed in claim 5, wherein the compositionfurther comprises at least one acidic organic compound.
 22. A polymerobtainable by the process as defined in claim 11, which comprises atleast two polymeric blocks.
 23. The process as claimed in claim 8, inwhich the at least one ethylenically unsaturated monomer is selectedfrom the group consisting of styrene, alpha-methylstyrene, vinyltoluene,vinyl halides, vinyl nitriles, conjugated dienes, unsaturated ethylenicmonomers containing a secondary, tertiary or quaternary amino group anda heterocyclic group containing nitrogen.
 24. The process as claimed inclaim 11, wherein the at least one ethylenically unsaturated monomer isselected from the group consisting of butadiene, chloroprene, styrene,α-methylstyrene, vinyltoluene, vinyl and vinylidene halides, vinylnitriles, vinyl pyridine and vinylimidazole.